The invention is directed to an apparatus for the measurement of the flow velocity of gases and liquids especially for the measurement of the intake air of combustion motors, having one or more temperature dependent resistances, of which one or more are heated, whereby the heated resistance or resistances are arranged behind a protective shield, referring to the direction of flow.
In the interest of maintaining the purity of the air it is necessary to reduce the pollutants produced by combustion engines, especially automobiles. This goal is attained among others by an improvement of the course of combustion. For this purpose it is necessary that the most exact information be available in each case in regard to the amounts of intake air, namely, with the smallest possible delay in time.
Hot wire anemometers of various construction have already been proposed for the measurement of the amount of intake air of combustion engines. The decisive disadvantage of customary hot wire anemometers is that the particles of dirt entrained in the intake air deposit on the heated resistance and through this change the measured characteristic of the heat emission. In order to prevent this it has been proposed to use as heated resistance a metal film resistance on a supporting plate and to so arrange this that the deposit of dust does not take place on the heat transmitting surface.
On the one hand this can be strived for by making the surface turned to the flow as small as possible compared to the heat transmitting surface.
Further possibilities are to thermally insulate the surface of the heated resistance turned to the flow or arrange the heated resistance behind a protective shield. The procedures mentioned, however, cause an instability of the flowing around the heated resistance which lead to fluctuations of the heat transmission and therewith the measured signal.
It has furthermore been proposed to arrange a flow barrier before the heated resistance which should lead to a stabile turbulent flow around the heated resistance. Provided that the flow barrier is suited to prevent the deposit of dirt on the heated resistance to the required extent it is not possible to obtain a sufficient stability of the turbulent flow behind this flow barrier. It turns out that the development of the flow behind the flow barrier to a considerable measure is dependent on the degree of turbulence of the flow before the barrier so that there results a severe dependence of the signal measured on disturbances at the air inlet.
Therefore, it was the problem of the present invention to devise an apparatus for the measurement of flow velocity of gases and liquids, especially the intake air of combustion engines, with one or more temperature dependent resistances, of which one or more are heated whereby the heated resistance or resistances are arranged behind a protective shield, referring to the direction of flow. This apparatus should have a stabile measured characteristic and be independent of disturbances at the air inlet in its output signal.